Polyester film

ABSTRACT

[Problems] The present invention provides a-polyester-film superior in heat resistance, chemical resistance, insulation property and thermal dimensional stability, and suitable for application to fields associated with boiling or retort treatment, which require tenacity, pinhole resistance, bending resistance, bag breakage resistance on dropping, impact resistance and the like, fields requiring thermoforming or vacuum forming, and various uses such as packaging bags for water-containing food, pharmaceutical products and the like. [Solving Means] The polyester film characteristically shows an initial elastic modulus in at least one direction of 2.5-10 GPa, an impact strength of 40-10000 J/mm, a thermal shrinkage in at least one direction at 150° C. of −0.5% to 6%, a haze of 0.001% to 7%, and an absolute value of the difference in the thermal shrinkage between the longitudinal direction and the transverse direction of not more than 1.1%.

TECHNICAL FIELD

The present invention relates to a polyester film superior in thermaldimensional stability and useful for various food packages, generalindustrial use, optical use, electric materials, mold processing use, aconstituent material of film laminated metal plate and the like.

BACKGROUND ART

Films made from polyester represented by polyethylene terephthalateresin have been extensively applied to various uses in view of themechanical property, heat resistance and the like. However, they are notsuitable for some uses because of the inferiority in flexibility andmolding processibility. On the other hand, since films made frompolyamide represented by 6-nylon are superior in flexibility, pinholeresistance and gas barrier property, they are applied to many uses suchas food packaging materials and the like. However, due to poordimensional stability against moisture absorption, they cannot be easilyapplied to food packaging uses and industrial uses involving boilingtreatment and retort treatment.

Therefore, a polyester film having flexibility, which is one of thefeatures of polyamide film, has been considered. Most of these filmsacquire flexibility based on the use of a polyester copolymer in a partor the entirety of the constituent resin. However, since they showdegraded strength and elastic modulus, which are mechanical properties,problems may occur during post-processing such as printing and the like.

In view of the above, a flexible film made from crystalline polyesterhas been studied and, for example, films comprising polyethyleneterephthalate resin and polybutylene terephthalate resin are known(Patent References 1, 2, 3). In these films, prevention of the incidenceof problems during post-processing has been tried by decreasing thermalshrinkage by conducting heat setting after completion of biaxialorientation. Since the difference in the melting points betweenpolyethylene terephthalate resin and polybutylene terephthalate resin isabout 30° C., heat shrinkage cannot be suppressed sufficiently, which inturn necessitates many limitations during post-processing.

Moreover, since these film are free of an easily adhesive coating layer,problems occur in that easily adhesive property of ink becomes degraded,stability of gas barrier property becomes defective after boilingtreatment and the like, since a vapor-deposited layer formed of metal orinorganic oxide shows poor adhesion and so forth. In addition, problemsof whitening and the like occur unless conditions for forming an easilyadhesive coating layer are optimized.

Patent Reference 1: JP-A-2002-037993

Patent Reference 2: JP-A-2002-179892

Patent Reference 3: JP-A-2002-321277

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made in view of the problems of theabove-mentioned conventional films, and provides a polyester filmsuperior in mechanical strength, heat resistance, chemical resistance,insulation property and thermal dimensional stability, and suitable forapplication to fields associated with boiling or retort treatment, whichrequire tenacity, pinhole resistance, bending resistance, bag breakageresistance on dropping, impact resistance and the like, fields requiringthermoforming or vacuum forming, and various uses such as packaging bagsfor water-containing food, pharmaceutical products and the like.

Means of Solving the Problems

To achieve the above-mentioned object, the polyester film of the presentinvention has an initial elastic modulus in at least one direction of2.5-10 GPa, an impact strength of 40-10000 J/mm, a thermal shrinkage inat least one direction at 150° C. of −0.5 to 6%, a haze of 0.001-7%, andan absolute value of the difference in the thermal shrinkage between thelongitudinal direction and the transverse direction of not more than1.1%.

In this case, the aforementioned polyester film may be made of apolyester resin composition comprising 10-90 wt % of polyethyleneterephthalate resin A, and 90-10 wt % of a polybutylene terephthalateresin and/or polytrimethylene terephthalate resin B.

In this case, the reduced viscosity of the aforementioned polyester filmmay be not less than 0.80.

In this case, moreover, the polyester film of claim 1, wherein theabsolute value of the difference in the thermal shrinkage between thelongitudinal direction and the transverse direction of theaforementioned substrate film is not more than 1.1%.

In this case, moreover, the polyester film of claim 1, wherein thethermal shrinkage in the longitudinal direction and the transversedirection at 150° C. of the aforementioned substrate film is each 0-4%.

In this case, moreover, the polyester film of claim 1, wherein thenumber of pinholes formed by bending the substrate film 1000 times at23° C. in a Gelbo-Flex test is not more than 5.

In this case, moreover, at least one surface treatment layer selectedfrom a coating layer, a corona discharge treatment layer, avapor-deposited metal layer, a vapor-deposited inorganic oxide layer andan ink printed layer can be formed on at least one surface of theaforementioned polyester film.

In this case, moreover, the polyester film of claim 6, wherein theaforementioned easily adhesive coating layer is composed of a coatingsolution containing at least binder (C) and hardener (D).

In this case, moreover, the polyester film of claim 6, which is obtainedby applying a coating solution for forming the aforementioned easilyadhesive coating layer, and then subjecting the resulting film to atleast uniaxial orientation.

In this case, moreover, the aforementioned polyester film can be used asa packaging material.

EFFECT OF THE INVENTION

The polyester film according to the present invention is superior inmechanical strength, heat resistance, chemical resistance, insulationproperty and thermal dimensional stability, and can be applied to fieldsassociated with boiling or retort treatment, which require tenacity,pinhole resistance, bending resistance, bag breakage resistance ondropping, impact resistance and the like, fields requiring thermoformingor vacuum forming, and various uses such as packaging bags forwater-containing food, pharmaceutical products and the like.

BEST MODE FOR EMBODYING THE INVENTION

The polyester film of the present invention has an initial elasticmodulus in at least one direction of 2.5-10 GPa, preferably 2.7-10 GPa,more preferably an initial elastic modulus in the longitudinal directionand the transverse direction of 2.7-10 GPa, particularly preferably2.7-5 GPa. When it is less than 2.5 GPa, the film may be broken duringhigh speed printing, printing displacement may occur and handling of thefilm in the form of a bag becomes difficult. When it exceeds 10 GPa, theproducibility of the film becomes poor.

The polyester film of the present invention shows an impact strength of40-10000 J/mm, preferably 60-1000 J/mm, more preferably 60-200 J/mm.When it is less than 40 J/mm, the film in the form of a bag filled withthe contents may gets broken when dropped and the like. When it exceeds10000 J/mm, the producibility of the film becomes poor.

The polyester film of the present invention has a thermal shrinkage inat least one direction of −0.5% to 6%, preferably 0% to 3%, morepreferably 0% to 1.5%. When it is lower than −0.5% or exceeds 6%,deformation of the film unpreferably occurs in the post-printing dryingand the like.

In addition, the polyester film of the present invention has a haze of0.001% to 7%, preferably 0.01% to 5%. A haze of less than 0.001% isdifficult to achieve in view of the production steps, and the productioncost becomes high. When the haze exceeds 7%, visual appearance becomesdefective after back printing, thus posing problems in design.

The polyester film of the present invention shows an absolute value ofthe difference in the thermal shrinkage between the longitudinaldirection and the transverse direction of not more than 1.1%, preferablynot more than 0.9%, more preferably not more than 0.6%, furtherpreferably not more than 0.3%. When it exceeds 1.1%, the gas barrierproperty after retort treatment may not be stabilized, or thermalstability may be lost, which is unpreferable. More preferably, thethermal shrinkage of the substrate film in the longitudinal directionand the transverse direction at 150° C. is 0% to 4% for each direction.

The polyester film of the present invention shows the number of pinholesof not more than 5, preferably not more than 2, more preferably 0, whichare formed by bending 1000 times at 23° C. in a Gelbo-Flex test. Whenthe number of pinholes is not less than 6 and when the film is processedinto a bag filled with the contents, pinholes due to bending of the bagand the like are easily developed, sometimes resulting in a failure tofunction as a packaging material.

The reduced viscosity (ηsp/c) of the polyester film of the presentinvention is preferably not less than 0.80, more preferably not lessthan 0.85, still more preferably not less than 0.90. When it is lessthan 0.80, the impact strength decreases, and when the film is processedinto a bag filled with the contents, bag breakage easily occurs.

The thickness of the polyester film of the present invention isgenerally 3-1000 μm, preferably 3-100 μm, more preferably 5-70 μm,particularly preferably 8-30 μm.

The polyester film of the present invention is preferably made of apolyester resin composition containing 10-90 wt % of polyethyleneterephthalate resin A, and 90-10 wt % of a polybutylene terephthalateresin and/or polytrimethylene terephthalate resin B. The polyester resincomposition contains polyethylene terephthalate resin A (hereinaftersometimes to be abbreviated as resin A) in a proportion of 10-90 wt %,preferably 15-70 wt %, more preferably 20-49 wt %, and a polybutyleneterephthalate resin and/or polytrimethylene terephthalate resin B(hereinafter sometimes to be abbreviated as resin B) in a proportion of90-10 wt %, preferably 85-30 wt %, more preferably 80-51 wt %. Whenresin A is contained in a proportion of less than 10 wt %,stretchability becomes poor due to the fast crystallization rate ofresin B, causing easy breakage during film forming, and when resin A iscontained in a proportion of more than 90 wt %, flexibility becomesinsufficient. When resin B is contained in a proportion of less than 10wt %, flexibility becomes insufficient, and when resin B exceeds 90 wt%, stretchability of the film becomes poor due to the fastcrystallization rate of resin B, causing easy breakage during filmforming.

The polyethylene terephthalate resin A to be used in the presentinvention preferably has a reduced viscosity of 0.55-1.20, morepreferably 0.55-0.80. When the reduced viscosity is smaller than thisrange, a film having practical mechanical strength cannot be obtainedeasily, and when it exceeds this range, the film forming property of thefilm is unpreferably degraded.

The polyethylene terephthalate resin A to be used in the presentinvention is preferably made of a homopolymer mainly comprisingterephthalic acid and ethylene glycol. As long as the heat resistanceand other properties (crystallinity etc.) are not impaired, it may be apolyester copolymer wherein not more than 20 mol %, preferably 0.1-10mol %, of a different acid component or a glycol component iscopolymerized.

When the polyethylene terephthalate resin to be used in the presentinvention is a copolymer, the following monomers can be used as acopolymerizable component.

As the dicarboxylic acid usable for copolymerization, aromaticdicarboxylic acid is exemplified by isophthalic acid, orthophthalicacid, naphthalene dicarboxylic acid, biphenyl dicarboxylic acid and thelike. Examples of aliphatic dicarboxylic acid include succinic acid,adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, dimer acidand the like, and examples of alicyclic dicarboxylic acid include1,4-cyclohexane dicarboxylic acid, 1,3-cyclohexane dicarboxylic acid,1,2-cyclohexane dicarboxylic acid, acid anhydride thereof and the like.Examples of dicarboxylic acid containing a polymerizable unsaturateddouble bond include α,β-unsaturated dicarboxylic acid (e.g., fumaricacid, maleic acid, maleic anhydride, itaconic acid, citraconic acid),alicyclic dicarboxylic acid containing an unsaturated double bond (e.g.,2,5-norbornene dicarboxylic anhydride, tetrahydrophthalic anhydride) andthe like.

As the glycol usable for copolymerization, aliphatic glycol having 3 to10 carbon atoms, alicyclic glycol having 6 to 12 carbon atoms and etherbond-containing glycol and the like can be mentioned. As the aliphaticglycol having 3 to 10 carbon atoms, 1,2-propylene glycol,1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, neopentyl glycol,1,6-hexanediol, 3-methyl-1,5-pentanediol, 1,9-nonanediol,2-ethyl-2-butylpropanediol and the like can be mentioned. As thealicyclic glycol having 6 to 12 carbon atoms, 1,4-cyclohexanedimethanoland the like can be mentioned.

Further, as the ether bond-containing glycol, polyethylene glycol,polypropylene glycol, polytetramethylene glycol, diethylene glycol,triethylene glycol, dipropylene glycol, glycols obtained by addingethyleneoxide or propyleneoxide to two phenolic hydroxyl groups ofbisphenols (e.g., 2,2-bis(4-hydroxyethoxyphenyl)propane) and the likecan be mentioned.

In addition, the polybutylene terephthalate resin and/orpolytrimethylene terephthalate resin B to be used in the presentinvention preferably has a reduced viscosity of 0.80-2.20. When theintrinsic viscosity is smaller than this range, a film having practicalmechanical strength cannot be obtained easily, and when it exceeds thisrange, the film forming property of the film is unpreferably degraded.

The polybutylene terephthalate resin to be used in the present inventionis preferably made of a homopolymer mainly comprising terephthalic acidand butanediol. As long as the heat resistance and other properties(crystallinity etc.) are not impaired, it may be a polyester copolymerwherein not more than 20 mol %, preferably not more than 10 mol %, of adifferent acid component or a glycol component is copolymerized. Inaddition, the polytrimethylene terephthalate resin is preferably made ofa homopolymer mainly comprising terephthalic acid and trimethyleneglycol. As long as the heat resistance and other properties are notimpaired, it may be a polyester copolymer wherein not more than 20 mol%, preferably 0.1-10 mol %, of a different acid component or a glycolcomponent is copolymerized.

When the polybutylene terephthalate resin or polytrimethyleneterephthalate resin to be used in the present invention is a copolymer,the following monomers can be used as a copolymerizable component.

As the dicarboxylic acid usable for copolymerization, variousdicarboxylic acids can be mentioned. Of these, examples of aromaticdicarboxylic acid include isophthalic acid, orthophthalic acid,naphthalene dicarboxylic acid, biphenyl dicarboxylic acid and the like.Examples of aliphatic dicarboxylic acid include succinic acid, adipicacid, azelaic acid, sebacic acid, dodecanedioic acid, dimer acid and thelike, and examples of alicyclic dicarboxylic acid include1,4-cyclohexane dicarboxylic acid, 1,3-cyclohexane dicarboxylic acid,1,2-cyclohexane dicarboxylic acid, acid anhydride thereof and the like.Examples of dicarboxylic acid containing a polymerizable unsaturateddouble bond include α,β-unsaturated dicarboxylic acid (e.g., fumaricacid, maleic acid, maleic anhydride, itaconic acid, citraconic acid),alicyclic dicarboxylic acid containing an unsaturated double bond (e.g.,2,5-norbornene dicarboxylic anhydride, tetrahydrophthalic anhydride) andthe like.

As the glycol usable for copolymerization, aliphatic glycol having 2 to10 carbon atoms, alicyclic glycol having 6 to 12 carbon atoms, etherbond-containing glycol and the like can be mentioned. As the aliphaticglycol having 2 to 10 carbon atoms, ethylene glycol, 1,2-propyleneglycol, trimethylene glycol, 1,4-butanediol, 1,5-pentanediol, neopentylglycol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, 1,9-nonanediol,2-ethyl-2-butylpropanediol and the like can be mentioned. As thealicyclic glycol having 6 to 12 carbon atoms, 1,4-cyclohexanedimethanoland the like can be mentioned.

Furthermore, as the ether bond-containing glycol, polyethylene glycol,polypropylene glycol, polytetramethylene glycol, diethylene glycol,triethylene glycol, dipropylene glycol, glycols obtained by addingethylene oxide or propylene oxide to two phenolic hydroxyl groups ofbisphenols (e.g., 2,2-bis(4-hydroxyethoxyphenyl)propane etc.) and thelike can be mentioned.

The polyester film of the present invention can contain any amount offine particles in a polyester resin composition constituting thepolyester film. For example, silicon dioxide, kaolin, clay, calciumcarbonate, calcium terephthalate, aluminum oxide, titanium oxide,calcium phosphate, silicone particles and the like can be mentioned,with preference given to inorganic lubricants. During melt mixing,additives such as stabilizer, coloring agent, antioxidant, antifoamingagent, antistatic agent and the like can be added as necessary besideslubricant.

The polyester film of the present invention should have mechanicalstrength. For this end, polyethylene terephthalate resin A, andpolybutylene terephthalate resin and/or polytrimethylene terephthalateresin B constituting the polyester film preferably do not allowcopolymerization during melt extrusion. As a method for suppressingcopolymerization, (1) a method comprising adding a particular phosphoruscompound to suppress transesterification reaction, (2) a methodcomprising controlling the size of the resin pellets to be mixed so asto prevent smooth mixing, (3) a method comprising lowering thetemperature of extruder to prevent easy progression oftransesterification, (4) a method comprising using an extruder having adouble flight type screw so as to prevent smooth mixing of resin A andresin B, (5) a method comprising melt extrusion of resin A and resin Bfrom two extruders each at a particular temperature, mixing them in amolten state immediately before extruding from a T-die and thenextruding the mixture from the T-die and the like can be mentioned.

When a phosphorus compound is added to suppress copolymerization ofpolyethylene terephthalate resin A and a polybutylene terephthalateresin and/or polytrimethylene terephthalate resin B during meltextrusion, it is preferable, but not limited, to use a phosphoruscompound having a melting point of not less than 200° C. and a molecularweight of not less than 200. While the optimal amount of addition variesdepending on the kind of the phosphorus compound, polymerizationconditions and the like, addition in a proportion of 0.01-0.3 wt % ispreferable for the suppression of transesterification reaction betweenresin A and resin B. For use for food such as beverage can and the like,the compound and amount thereof should meet the standard of FDA (U.S.Food and Drug Administration), Japan Hygienic Olefin and StyrenePlastics Association and the like. To increase crystallization rate, itis also preferable to add a substance to be a nucleating agent forcrystallization, while suppressing copolymerization.

The substance to be a nucleating agent for crystallization can be addedafter mixing with a plasticizer such as polyethylene, polypropylene,polystyrene, polyester polymer or polyester copolymer having a molecularweight of not more than 20000, which has the aforementioned monomerconstitution, fatty acid ester and the like. As the nucleating agent forcrystallization, inorganic particles of calcium carbonate,non-crystalline zeolite particles, anatase type titanium dioxide, rutiletype titanium dioxide, calcium phosphate, silica, kaolin, talc, clay,barium sulfate, zinc oxide, zinc sulfide and the like can be mentioned,which are generally added in a proportion of 0.001-2 wt %, preferably0.01-1 wt %. However, the results thereof vary drastically depending onthe substances to be added, the amount of addition, means of addition,order of addition, particle size and the like, as well as the meltextrusion conditions of the film. Thus, for stable expression of theeffect, it is preferable to employ a method comprising preparing masterbatch pellets by the addition of inorganic particles duringpolymerization to a polyester copolymer having a molecular weight of notmore than 20000 and having the aforementioned monomer constitution, dryblending the pellets with at least the pellets of resin A and resin B tobe the substrate or master batch pellets, and melting and extruding themixture. As compared to the addition of inorganic particles to resin Aand resin B during polymerization, dispersibility in a resin mixturebecomes fine, which, it is considered, in turn increases crystallizationrate and suppresses whitening of polyester film during boiling, retorttreatment and thermoforming.

In addition, it is preferable to control the size of the resin pelletsto be mixed, so as to suppress copolymerization of the polyethyleneterephthalate resin A, and a polybutylene terephthalate resin and/orpolytrimethylene terephthalate resin B during melt extrusion.

It is also preferable to set the temperature of each part to not morethan 270° C., preferably not more than 262° C., during the period offrom mixing and melting of the resins, passage through an extruder, toextrusion from a T-die, so as to suppress copolymerization of thepolyethylene terephthalate resin A, and a polybutylene terephthalateresin and/or polytrimethylene terephthalate resin B during meltextrusion. The presence of a temperature zone exceeding 270° C. in amelt extrusion step is considered to accelerate decomposition of apolyester usable as resin B, which in turn promote copolymerization ofresin A and resin B. When the film of the present invention is to beproduced using a single extruder, in the melt extrusion step, resin A,resin B and other resin pellets constituting the aforementionedpolyester resin composition are mixed, cast in an extruder, melted,extruded from a T-die, adhered to a cooling roll by an electrostaticadhesion method and the like, and solidified by cooling to give anon-oriented sheet. In this case, the temperature of an extruder ispreferably set to not more than 270° C., preferably not more than 262°C., for all of the feeding part, compression part, measuring part,filter, resin flow path and T-die of the extruder.

In general, when the extrusion temperature conditions for resin aredescribed in literatures, the resin temperature immediately before entryof the resin into a T-die or immediately after delivery of the resinfrom the T-die is often taken as the temperature condition of extrusion.Control of only the resin temperature immediately before entry into aT-die or immediately after delivery from the T-die is insufficient todefinitely control the resin temperature during the melt extrusion stepup to the T-die. It is a general practice to intentionally change thefeeding part, compressing part, measuring part, filter and resin flowpath of an extruder, in view of the specific condition of the machine,such as screw shape of the extruder and the like, production speed andstability, and the temperature of the respective parts is in fact oftendifferent from each other.

Furthermore, when the film of the present invention is to be producedusing a single extruder, so as to suppress copolymerization ofpolyethylene terephthalate resin A, and a polybutylene terephthalateresin and/or polytrimethylene terephthalate resin B during meltextrusion, a uniaxial extruder having a double flight type screwcompression part (compression zone), which is of a rapid compressiontype with a small compression ratio (not more than 2.0) is preferablyused. As an extruder having a double flight type screw, UB seriesmanufactured by Mitsubishi Heavy Industries, Ltd. can be mentioned.

To suppress copolymerization of polyethylene terephthalate resin A, anda polybutylene terephthalate resin and/or polytrimethylene terephthalateresin B during melt extrusion, moreover, resin A and resin B arerespectively melted and extruded from two extruders, mixed in a moltenstate and immediately thereafter extruded from a T-die to give the filmof the present invention. As a method for mixing in a molten stateimmediately before extrusion from a T-die, a method comprising feedingrespective molten resins in a molten state to molten resin mixers suchas extruders, static mixers and the like, melt-mixing and extrudingthem, and the like can be mentioned. As a device for mixing moltenresins, conventional uniaxial extruder, biaxial extruder, dynamic mixer,static mixer (manufactured by Noritake Co., Limited and the like) andthe like can be mentioned. As a preferable method for suppressingcopolymerization of polyethylene terephthalate resin A andpolytrimethylene terephthalate resin B during melt extrusion, theabove-mentioned method (5), and a combination of method (5) and othermethod can be mentioned.

While the polyester film of the present invention can be used when itmeets the requirements of a non-oriented sheet of the present invention,it can be preferably obtained by orienting a non-oriented sheet at leastuniaxially, more preferably biaxially or more. The method of orientationincludes tubular orientation, pantographic simultaneous biaxialorientation, linear motor simultaneous or sequential biaxialorientation, sequential biaxial orientation based on a combination of aheating roll and a tenter and the like. In the case of sequentialbiaxial orientation, orientation methods of longitudinal-transverse,transverse-longitudinal, longitudinal-longitudinal-transverse,longitudinal-transverse-longitudinal,longitudinal-transverse-transverse,longitudinal-longitudinal-longitudinal-transverse and the like can bementioned.

The production method of the film of the present invention is shownbelow by referring to a sequential biaxial orientation method as anexample. Resin chips of polyethylene terephthalate resin A, and apolybutylene terephthalate resin and/or polytrimethylene terephthalateresin B are mixed, cast into one extruder, melted, mixed and extruded.The mixture is extruded from a T-die and the melt extruded sheet iselectrostatically adhered to a chill roll to give a non-oriented sheet.The temperature of the chill roll then is preferably 10-40° C. When thetemperature of the chill roll exceeds 40° C., the polybutyleneterephthalate resin and/or polytrimethylene terephthalate resin Bcrystallize to whiten the extruded sheet. As a result, haze afterbiaxial orientation becomes poor and the printed layer becomes difficultto see.

The obtained non-oriented sheet is delivered to a pair of orientingrolls at 50-100° C. with different speeds, drawn 2.5 to 5-fold in thelongitudinal direction. The longitudinally oriented sheet is deliveredto a tenter and drawn 2.5 to 5-fold in the transverse direction at60-120° C. Where necessary, the sheet may be drawn in the longitudinaldirection and the transverse direction in two steps at differenttemperature and different folding rate. Then, the sheet is heat set at180-230° C. to give a polyester film.

The polyester film of the present invention shows not more than 1.1% ofthe difference in the absolute value of thermal shrinkage between thelongitudinal direction and the transverse direction. To obtain such apolyester film, the film forming conditions are controlled. According toa conventional sequential biaxial orientation method, thermal shrinkageis suppressed by heat setting in situ in the tenter after completion ofthe second axial orientation, while relaxing the sheet in the transversedirection. However, since relaxing in the longitudinal direction at thispoint is difficult, thermal shrinkage in the longitudinal direction andthat in the transverse direction often becomes different. In the presentinvention, a method of relaxing in the longitudinal direction with theclips of the tenter, or a method of relaxing the sheet after leaving thetenter using heating rolls having different rotation speeds ispreferably employed. Since the relaxing conditions vary depending on theorientation ratio, speed and the like, individual conditions aredetermined. Generally preferably, the heat setting temperature is180-230° C., the relaxing rate is 2-8%, the difference in the heatsetting temperature between the longitudinal direction and thetransverse direction is not more than 20° C., and the difference in therelaxing rate between the longitudinal direction and the transversedirection is within 2% (e.g., when the relaxing rate in the transversedirection is set for 4% that in the longitudinal direction is set to2-6%).

The substrate film used for forming the polyester film of the presentinvention preferably has an easily adhesive coating layer, preferably aneasily adhesive coating layer comprising binder C and hardener D, on atleast one surface thereof. When an easily adhesive coating layer is notformed, adhesiveness of the printing ink may be degraded, and adhesionof a metal or inorganic oxide vapor-deposited layer becomesinsufficient, thus degrading the stability of gas barrier property.Particularly, when bags filled with food or liquid are sealed, and manyof such bags are boil-treated with circulation during a boilingtreatment, a vapor-deposited layer is easily detached in the absence ofan easily adhesive coating layer, and the gas barrier property becomesunstable.

The polyester film of the present invention can have at least onesurface treatment layer selected from a surface activation treatedlayer, a vapor-deposited metal layer, a vapor-deposited inorganic oxidelayer and a printed ink layer on at least one surface thereof, besidesthe coating layer or on the coating layer. In the present invention,these layers may be formed on one surface of the film or both surfacesthereof, which can be appropriately determined according to use. Theabsolute value of the difference in the thermal shrinkage between thelongitudinal direction and the transverse direction of the polyesterfilm having such a surface treatment layer is not more than 1.1%,preferably not more than 0.9%, more preferably not more than 0.6%, andstill more preferably not more than 0.3%. When it exceeds 1.1%, gasbarrier property after retort treatment may become unstable and thethermal stability may be lost, which are unpreferable.

In the present invention, a coating agent for forming a coating layer onat least one surface of a polyester film is not particularly limited aslong as it has adhesiveness. Examples thereof include coating agentsmade of polyester resin, polyurethane resin, polyacryl resin, polyvinylalcohol resin and copolymers thereof, ethylene-vinyl acetate copolymerresin and the like. To improve adhesion between a polyester film andink, a vapor-deposited layer and the like, a polyester resin coatingagent is preferably used. One-component or two-component polyurethaneresin coating agents are also preferable. Specific examples of thetwo-component polyurethane coating agent include product name TakerakA2027 and Takenate A3 (both manufactured by Takeda PharmaceuticalCompany Ltd.). As the polyester resin coating agent, product name VYLON(manufactured by Toyo Boseki Kabushiki Kaisha) and a water-insolublepolyester copolymer obtained by reacting mixed dicarboxylic acid ofmetal sulfonate group-containing dicarboxylic acid (0.5-15 mol %) anddicarboxylic acid free of metal sulfonate group (85-99.5 mol %) with apolyol component can be mentioned. As the above-mentioned metalsulfonate group-containing dicarboxylic acid, metal salts such as5-sulfoisophthalic acid, 4-sulfophthalic acid,4-sulfonaphthalene-2,7-dicarboxylic acid, 5[4-sulfophenoxy]isophthalicacid and the like can be mentioned. Particularly preferred are sodium5-sulfoisophthalate and sodium sulfoterephthalate. The metal sulfonategroup-containing dicarboxylic acid component is used in a proportion of0.5-15 mol %, desirably 2.0-10 mol %, relative to the entiredicarboxylic acid component. When it exceeds 15 mol %, dispersibility inwater is improved but water resistance of polyester copolymer ismarkedly degraded, and when it is less than 0.5 mol %, dispersibility inwater is markedly degraded. While the dispersibility of the polyestercopolymer in water varies depending on the kind, mixing ratio and thelike of the copolymerization component, the above-mentioned metalsulfonate group-containing dicarboxylic acid is preferably used in asmall amount as long as its dispersibility in water is not impaired. Asthe dicarboxylic acid free of metal sulfonate group, aromatic, alicyclicand aliphatic dicarboxylic acids can be used. As the aromaticdicarboxylic acid, terephthalic acid, isophthalic acid, orthophthalicacid, 2,6-naphthalene dicarboxylic acid and the like can be mentioned.The aromatic dicarboxylic acid is preferably used in a proportion of notless than 40 mol % of the entire dicarboxylic acid component. When it isless than 40 mol %, the mechanical strength and water resistance of thepolyester copolymer decreases. As the aliphatic and alicyclicdicarboxylic acids, succinic acid, adipic acid, sebacic acid,1,3-cyclohexane dicarboxylic acid, 1,4-cyclohexane dicarboxylic acid andthe like can be mentioned. Addition of these non-aromatic dicarboxylicacid component sometimes results in improved adhesiveness, but itgenerally degrades mechanical strength and water resistance of thepolyester copolymer. The polyol component to be reacted with theabove-mentioned mixed dicarboxylic acid is aliphatic glycol having 2 to8 carbon atoms or alicyclic glycol having 6 to 12 carbon atoms. Specificexamples thereof include ethylene glycol, 1,2-propylene glycol,1,3-propanediol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol,1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol,1,4-cyclohexanedimethanol, P-xylylene glycol, diethylene glycol,triethylene glycol and the like. As polyether, polyethylene glycol,polypropylene glycol, polytetramethylene glycol and the like can bementioned. In addition, an oxycarboxylic acid component such asp-oxyethoxybenzoic acid may be copolymerized.

As the polyester resin, a polyester graft copolymer can also be used. Inthe present invention, the “graft formation” means introduction of abranch polymer made of a polymer other than the main chain into a branchpolymer main chain.

(Polyester Graft Copolymer)

The graft polymerization is generally carried out by reacting ahydrophobic copolymerizable polyester resin dissolved in an organicsolvent with at least one kind of polymerizable unsaturated monomerusing a radical initiator. The reaction product after completion of thegrafting reaction includes a desired graft copolymer of hydrophobiccopolymerizable polyester and polymerizable unsaturated monomer,hydrophobic copolymerizable polyester resin free from graft formationand the above-mentioned unsaturated monomer polymer not grafted to thehydrophobic copolymerizable polyester. The polyester graft copolymer inthe present invention means not only the above-mentioned polyester graftcopolymer but also a reaction mixture containing hydrophobiccopolymerizable polyester unreactive therewith, polymer of unsaturatedmonomer not grafted thereto and the like.

In the present invention, polyester graft copolymer obtained by graftpolymerization of hydrophobic copolymerizable polyester resin and atleast one kind of polymerizable unsaturated monomer preferably has anacid value of not less than 600 eq/10⁶ g, more preferably not less than1200 eq/10⁶ g. When the graft copolymer has an acid value of less than600 eq/10⁶ g, adhesion of the object graft copolymer-containing layer ofthe present invention to a layer covered therewith does not becomesufficient.

The weight ratio of the hydrophobic copolymerizable polyester resin andthe polymerizable unsaturated monomer, which affords a desirable graftcopolymer is desirably within the range of polyester/polymerizableunsaturated monomer=40/60-95/5, more desirably 55/45-93/7, mostdesirably 60/40-90/10. When the weight ratio of the hydrophobiccopolymerizable polyester resin is less than 40 wt %, superioradhesiveness of the polyester resin cannot be exhibited. On the otherhand, when the weight ratio of the hydrophobic copolymerizable polyesterresin is greater than 95 wt %, the defect of the polyester resin, i.e.,blocking, easily occurs.

The polyester graft copolymer to be used in the present invention is inthe form of a solution or dispersion in an organic solvent, or asolution or dispersion in an aqueous solvent. Particularly, a dispersionin an aqueous solvent, namely, an aqueous resin dispersion, ispreferable in terms of work environment and coatability. Such aqueousdispersion resin can be generally obtained by graft polymerization ofthe aforementioned hydrophobic copolymerizable polyester resin with atleast one kind of hydrophilic polymerizable unsaturated monomer in anorganic solvent, adding water and evaporating the organic solvent.

The above-mentioned polyester graft copolymer preferably shows anaverage particle size of not more than 500 nm, particularly 10-500 nm,as measured by a laser scattering method, and a semitransparent toopalescent appearance. Graft copolymers having various particle sizescan be obtained by controlling the polymerization method. The averageparticle size is preferably not more than 400 nm, more preferably notmore than 300 nm, from the aspect of dispersion stability. When itexceeds 500 nm, the gloss of the surface of a coated film is degradedand so is the transparency. When it is less than 10 nm, the object waterresistance of the present invention is unpreferably degraded.

The polymerizable unsaturated monomer grafted to a hydrophobiccopolymerizable polyester resin is a hydrophilic radical polymerizablemonomer, which has a hydrophilic group or a group capable converting toa hydrophilic group later. As the hydrophilic group, carboxyl group,hydroxyl group, phosphoric acid group, phosphorous acid group, sulfonicacid group, amide group, quaternary ammonium salt group and the like canbe mentioned. As a group capable of converting to a hydrophilic group,acid anhydride group, glycidyl group, chloro group and the like can bementioned. Of these groups, a carboxyl group is preferable in view ofthe aqueous dispersibility and increased acid value of a graftcopolymer. Thus, a polymerizable unsaturated monomer having a carboxylgroup or a group capable of converting to a carboxyl group ispreferable.

The glass transition temperature of the polyester graft copolymer is notmore than 30° C., preferably not more than 10° C. Use of a polyestergraft copolymer having a glass transition temperature of not more than30° C. for the graft copolymer-containing layer affords a polyester filmsuperior in adhesiveness. When the properties of the graft copolymer areoutside the above-mentioned range, the effect of a graftcopolymer-containing layer comprising a graft copolymer is not easilyexhibited.

(Hydrophobic Copolymerizable Polyester Resin)

In the present invention, the hydrophobic copolymerizable polyesterresin should be essentially water insoluble, which means that the resindoes not disperse or dissolve in water by itself. When a polyester resinthat is dispersed or dissolved in water is used for graftpolymerization, the object adhesiveness and water resistance of thepresent invention are degraded. The composition of the dicarboxylic acidcomponent of the hydrophobic copolymerizable polyester resin ispreferably aromatic dicarboxylic acid 60-99.5 mol % aliphaticdicarboxylic acid and/or alicyclic dicarboxylic acid 0-40 mol %,dicarboxylic acid having a polymerizable unsaturated double bond 0.5-10mol %. When the proportion of the aromatic dicarboxylic acid is lessthan 60 mol % or aliphatic dicarboxylic acid and/or alicyclicdicarboxylic acid are/is more than 40 mol %, the adhesion strengthdecreases.

When the proportion of the dicarboxylic acid having a polymerizableunsaturated double bond is less than 0.5 mol %, grafting of apolymerizable unsaturated monomer to a hydrophobic copolymerizablepolyester resin does not proceed efficiently, and when it exceeds 10 mol%, the viscosity markedly increases after grafting reaction tounpreferably prevents uniform progress of the reaction. More preferably,the proportion of the aromatic dicarboxylic acid is 70-98 mol %, theproportion of the aliphatic dicarboxylic acid and/or alicyclicdicarboxylic acid is 0-30 mol %, and the proportion of the dicarboxylicacid having a polymerizable unsaturated double bond is 2-7 mol %.

The urethane resin U usable in the present invention as binder C forforming an easily adhesive coating layer is heat reactive water-solubleurethane wherein a terminal isocyanate group is blocked with ahydrophilic group. As blocking agent for isocyanate group, a number ofcompounds such as bisulphites, and phenols, alcohols, lactams, oximesand active methylene compounds, each containing a sulfone group, and thelike can be used. The blocked isocyanate group can be removed by makingthe urethane prepolymer hydrophilic or water soluble, and the blockingagent can be removed by drying or heat setting during film productionand the like. When thermal energy is applied to the resin U having ablocked isocyanate group, the blocking agent is released from theisocyanate group and the resin U is self-crosslinked. Since resin U usedfor preparing a coating solution is hydrophilic, water resistancebecomes poor. However, after coating, drying and heat setting tocomplete the thermal reaction, a coated film having fine waterresistance can be obtained since the hydrophilic group of the urethaneresin U is released. Of the above-mentioned blocking agents, one havingadequate heat treatment temperature and heat treatment time andpermitting industrial wide use, bisulphites are preferable. As thechemical composition of a urethane prepolymer usable for theabove-mentioned resin U, (1) a compound having not less than two activehydrogen atoms in a molecule and a molecular weight of 200-20,000, (2)organic polyisocyanate having not less than two isocyanate groups in amolecule, and, in some cases, (3) a compound having a terminalisocyanate group obtained by reacting a chain extender having at leasttwo active hydrogen atoms in a molecule can be mentioned. As thecompound of the above-mentioned (1), compounds having not less than twohydroxyl groups, carboxyl groups, amino groups or mercapto groups in theterminal or in a molecule are generally known, and as particularlypreferable compounds, polyether polyol, polyester polyol, polyetheresterpolyol and the like can be mentioned. As the polyether polyol, forexample, alkylene oxides (e.g., ethylene oxide and propylene oxide), acompound wherein styrene oxide, epichlorohydrin and the like arepolymerized, a compound obtained by random copolymerization, blockcopolymerization or addition polymerization to polyol thereof, and thelike can be mentioned. As the polyester polyol and polyetheresterpolyol, linear or branched compounds can be mainly mentioned. They canbe obtained by condensation with succinic acid, adipic acid, phthalicacid, acid anhydride and the like, saturated or unsaturated alcohols(e.g., ethylene glycol, diethylene glycol, 1,4-butanediol, neopentylglycol, 1,6-hexanediol, trimethylolpropane etc.), polyalkyleneetherglycols having relatively low molecular weight (e.g., polyethyleneglycol, polypropylene glycol etc.), or mixtures thereof. In addition,polyesters obtained from lactone and hydroxy acid can also be used aspolyester polyol, and polyether esters obtained by adding ethyleneoxide, propylene oxide and the like to polyesters produced in advancecan also be used as polyetherester polyol. As the organic polyisocyanateof the above-mentioned (2), isomers of toluylene diisocyanate, aromaticdiisocyanates (e.g., 4,4-diphenylmethane diisocyanate etc.), aromaticaliphatic diisocyanates (e.g., xylylene diisocyanate etc.), alicyclicdiisocyanates (e.g., isophorone diisocyanate, 4,4-dicyclohexylmethanediisocyanate etc.), aliphatic diisocyanates (e.g., hexamethylenediisocyanate, 2,2,4-trimethylhexamethylene diisocyanate etc.), andpolyisocyanates obtained by addition of trimethylolpropane and the liketo these compounds (single or multiple) can be mentioned. As the chainextender having at least two active hydrogens of the above-mentioned(3), glycols (e.g., ethylene glycol, diethylene glycol, 1,4-butanediol,1,6-hexanediol etc.), polyols (e.g., glycerol, trimethylolpropane,pentaerythritol etc.), diamines (e.g., ethylenediamine,hexamethylenediamine, piperazine etc.), amino alcohols (e.g.,monoethanolamine, diethanolamine etc.), thiodiglycols (e.g.,thiodiethylene glycol etc.), and water can be mentioned.

In addition, the polyacryl resin can be obtained by polymerization ofacrylic acid or a derivative thereof and, where necessary, a monomerother than acrylic acid (derivative) and having a vinyl group. As themonomer to be used, for example, acrylic acid, methacrylic acid(hereinafter (meth)acrylic acid includes acrylic acid and/or methacrylicacid), lower alkyl ester of (meth)acrylic acid (e.g., methyl, ethyl,propyl, butyl, amyl, hexyl, heptyl, octyl, 2-ethylhexyl ester), methylmethacrylate, hydroxymethyl acrylate, styrene, glycidyl methacrylate,methyl acrylate, ethyl acrylate and the like can be mentioned.

In the present invention, the easily adhesive coating layer preferablycontains hardener D. As the hardener D, a phenol formaldehyde resinwhich is a condensate of alkylated phenols with formaldehyde, cresolsetc. with formaldehyde; addition product of urea, melamine,benzoguanamine etc. with formaldehyde, an amino resin comprising theaddition product and an alkyl ether compound comprising an alcoholhaving 1 to 6 carbon atoms; a multifunctional epoxy compound; amultifunctional isocyanate compound; a block isocyanate compound; amultifunctional aziridine compound; an oxazoline compound and the likecan be used. As the phenol formaldehyde resin, for example, condensatesof phenols such as alkylated (methyl, ethyl, propyl, isopropyl or butyl)phenol, p-tert-amylphenol, 4,4′-sec-butylidenephenol,p-tert-butylphenol, o-, m-, p-cresol, p-cyclohexylphenol,4,4′-isopropylidenephenol, p-nonylphenol, p-octylphenol,3-pentadecylphenol, phenol, phenylo-cresol, p-phenylphenol, xylenol andthe like and formaldehyde can be mentioned.

As the amino resin, for example, methoxymethylol urea, methoxymethylolN,N-ethyleneurea, methoxymethylol dicianediamide, methoxymethylolmelamine, methoxymethylol benzoguanamine, butoxymethylol melamine,butoxymethylol benzoguanamine and the like can be mentioned, withpreference given to methoxymethylol melamine, butoxymethylol melamine,methylol benzoguanamine and the like can be mentioned.

As the multifunctional epoxy compound, for example, diglycidyl ether ofbisphenol A and oligomer thereof, diglycidyl ether of hydrogenatedbisphenol A and oligomer thereof, diglycidyl orthophthalate, diglycidylisophthalate, diglycidyl terephthalate, diglycidyl p-oxybenzoate,diglycidyl tetrahydrophthalate, diglycidyl hexahydrophthalate,diglycidyl succinate, diglycidyl adipate, diglycidyl sebacate,ethyleneglycol diglycidyl ether, propyleneglycol diglycidyl ether,1,4-butanediol diglycidyl ether, 1,6-hexanediol diglycidyl ether andpolyalkyleneglycol diglycidyl ethers, triglycidyl trimellitate,triglycidylisocyanurate, 1,4-diglycidyloxybenzene,diglycidylpropyleneurea, glycerol triglycidyl ether, trimethylolpropanetriglycidyl ether, pentaerythritol triglycidyl ether, triglycidyl etherof glycerolalkyleneoxide adduct and the like can be mentioned.

As the multifunctional isocyanate compound, low molecular weight or highmolecular weight aromatic or aliphatic diisocyanate, and polyisocyanateof trivalent or more can be used. As the polyisocyanate, tetramethylenediisocyanate, hexamethylene diisocyanate, toluene diisocyanate,diphenylmethane diisocyanate, hydrogenated diphenylmethane diisocyanate,xylylene diisocyanate, hydrogenated xylylene diisocyanate, isophoronediisocyanate, and trimers of these isocyanate compounds can bementioned. In addition, a terminal isocyanate group-containing compoundobtained by reacting the above isocyanate compound in an excess amountand a low molecular weight active hydrogen compound (e.g., ethyleneglycol, propylene glycol, trimethylolpropane, glycerol, sorbitol,ethylenediamine, monoethanolamine, diethanolamine, triethanolamine etc.)or a high molecular weight active hydrogen compound such aspolyesterpolyols, polyetherpolyols, polyamides and the like can bementioned.

The blocked isocyanate can be prepared by addition reaction of theabove-mentioned isocyanate compound and a blocking agent by aconventionally known appropriate method. As the isocyanate blockingagent, for example, phenols such as phenol, cresol, xylenol, resorcinol,nitrophenol, chlorophenol etc.; thiophenols such as thiophenol,methylthiophenol etc.; oximes such as acetoxime, methylethylketoxime,cyclohexanoneoxime etc.; alcohols such as methanol, ethanol, propanol,butanol etc.; halogen-substituted alcohols such as ethylenechlorohydrin,1,3-dichloro-2-propanol etc.; tertiary alcohols such as t-butanol,t-pentanol etc.; lactams such as ε-caprolactam, δ-valerolactam,ν-butyrolactam, β-propyllactam etc.; aromatic amines; imides; activemethylene compounds such as acetylacetone, acetoacetic ester, ethylmalonate etc.; mercaptans; imines; ureas; diaryl compounds; sodiumbisulfite and the like can be mentioned.

These crosslinking agents D can be used alone or a combination of two ormore kinds thereof. The amount of the crosslinking agent D to be addedis preferably 10-150 parts by weight, more preferably 20-120 parts byweight, per 100 parts by weights of binder C. When it is less than 10parts by weight, a sufficient crosslinking effect is not expressed.Therefore, delamination and breakage of vapor-deposited film occurduring boiling treatment and the gas barrier property after boilingtreatment becomes defective. When it exceeds 150 parts by weight,drawing after coating becomes difficult and a film having an easilyadhesive coating layer becomes difficult to obtain.

In the present invention, the easily adhesive coating layer may containparticles, antistatic agent, surfactant, antioxidant, light shieldingagent, antigelling agent and the like.

In the present invention, to prevent degradation of water resistance ofthe easily adhesive coating layer and gas barrier property after boilingtreatment due to delamination and breakage of vapor-deposited filmduring boiling treatment, which is caused by hardener D orself-crosslinking type resin, sufficient heat needs to be applied toperform crosslinking. However, when a film after the completion ofbiaxial orientation is to be coated, the heat sufficient forcrosslinking is difficult to apply in view of the stability of asubstrate film used for forming a polyester film. Therefore, coatingwithin the film forming step is preferable, where crosslinking can becarried out simultaneously with the heat setting of the film, and amethod comprising coating a sheet after uniaxial orientation is morepreferable.

In the present invention, preferable combination of binder C andcrosslinking agent D is a polyester resin copolymer and a melamineresin, a polyvinyl alcohol resin and a melamine resin, a polyester resincopolymer or an isocyanate resin, which is contained at least in theeasily adhesive coating layer. A polyester graft copolymer that performsself-crosslinking can be used without a crosslinking agent, and ispreferable in view of producibility, blocking resistance and slipperyproperty.

In the present invention, to simultaneously improve haze, film formingstability and water resistance of the easily adhesive coating layer andgas barrier property thereof after boiling treatment, which may beimpaired by delamination and breakage of a vapor-deposited film duringboiling treatment, a coating solution applied to a uniaxial orientationsheet is preferably led to a tenter after drying. Since the polybutyleneterephthalate resin and the polytriethylene terephthalate resin show aslower crystallization rate as compared to a polyethylene terephthalateresin, the transverse orientation temperature and the preheatingtemperature immediately before that need to be set higher, when thecoating solution is led to a tenter without drying, which causes easyoccurrence of whitening due to crystallization (high haze) and breakageof the film. Therefore, an easily adhesive coating layer is preferablydried at 40-100° C. after coating. When it exceeds 100° C.,crystallization of a uniaxial orientation sheet becomes noticeable andwhitening and breakage are often caused. When it is less than 40° C.,drying tends to become insufficient.

In addition, and the rate of hot air is preferably controlled inconsideration of the producibility, and the stability of an easilyadhesive coating layer. While it depends on the machine and a filmforming rate, a hot air is preferably applied at 15-25 m/sec.

The substrate film used for forming the polyester film of the presentinvention has a reduced viscosity of preferably 0.70-1.50, morepreferably 0.80-1.10. When it is less than 0.70, insufficientflexibility leads to easy breakage of the vapor-deposited layer of metalor inorganic oxide during practical use and the gas barrier lacksstability. When it exceeds 1.50, breakage unpreferably occurs frequentlyin the orientation step of the film.

In the present invention, a coating layer can contain fine particles,antistatic agent, surfactant, antioxidant, light shielding agent,antigelling agent and the like. As the fine particles, for example,inorganic particles having a particle size of about 0.01-10 μm, such ascalcium carbonate, sedimentary barium carbonate, silica, talc and thelike, and organic particles such as polystyrene, polyester, melamine,benzoguanamine, acrylic particles and the like can be mentioned.

In the present invention, when a coating layer is to be formed on atleast one surface of a polyester film, a preferable method preferablycomprises applying a coating solution to a uniaxially oriented sheet,and feeding the sheet to a tenter and drawing the sheet in theperpendicular direction, thereby simultaneously preventing degradationof film forming stability and transparency, maintaining water resistanceof the coating layer, preventing delamination and breakage ofvapor-deposited film during boiling treatment, and preventingdegradation of gas barrier property after boiling treatment. In thiscase, it is preferable to apply a coating solution to a uniaxiallyoriented sheet, dry the sheet and then lead the sheet to a tenter. Sincethe polybutylene terephthalate resin and the polytrimethyleneterephthalate resin show a faster crystallization rate as compared to apolyethylene terephthalate resin, the transverse orientation temperatureand the preheating temperature before that need to be set higher, whenthe coating solution is led to a tenter without drying, which causeseasy occurrence of whitening due to crystallization (high haze) andbreakage of the film. Therefore, a coating layer is preferably dried atnot less than 40° C. and not more than 70° C. after coating. When itexceeds 70° C., crystallization of uniaxially oriented sheet begins,which easily cause whitening and breakage. When it is less than 40° C.,drying tends to become insufficient.

As a method for forming a coating layer in the present invention,conventional coating methods can be used. For example, gravure coating,micro gravure coating, bar coating, reverse roll coating, reversekissroll coating, comma coating, dam coating, curtain coating, dipcoating, blade coating and the like can be employed.

In addition, a surface activation treated (e.g., corona dischargetreatment) layer can be formed on at least one surface of the polyesterfilm of the present invention. Since such surface modification meansmodification of the polar group on the film surface, the wettingproperty of the film surface can be improved, and when the wettingtension of the surface of the surface activation treated layer is set tonot less than 35 dyne/cm, ink absence during printing can be reduced.

A vapor-deposited metal layer can be formed on at least one surface ofthe polyester film of the present invention. As a metal preferable forforming a vapor-deposited metal layer, aluminum, palladium, zinc,nickel, gold, silver, copper, indium, tin, chrome, titanium and the likecan be mentioned. Representatively, aluminum is used.

A vapor-deposited inorganic oxide layer can be formed on at least onesurface of the polyester film of the present invention. As an inorganicoxide preferable for forming a vapor-deposited inorganic oxide layer inthe present invention, any can be used as long as a vapor-depositedlayer of an. inorganic oxide, which shows transparency and gas barrierproperty, can be formed. Metal oxides and nonmetal oxides are widelyused, and particularly, a vapor-deposited layer comprising silicon oxideand/or aluminum oxide as main components is preferable.

While the film thickness of the vapor-deposited layer of a metal orinorganic oxide is not limited and may be any, it is within the range ofgenerally 10-5000 Å, more preferably 50-2000 Å.

A vapor-deposited layer of a metal or inorganic oxide can be formed byphysical vapor deposition methods such as vacuum vapor deposition,sputtering, ion plating and the like, chemical vapor deposition methodssuch as CVD and the like, and the like as appropriate. As a heatingmethod employed here, resistance heating, induction heating, electronbeam heating and the like can be employed as appropriate. As a reactiongas, oxygen, nitrogen, hydrogen, argon, carbon gas, water vapor and thelike can be introduced, or reactive vapor deposition using ozoneaddition, ion assist etc. may be employed. In addition, bias may beapplied to a substrate, and film forming conditions such as heating andcooling of a substrate, may be changed. Such vapor deposition material,reaction gas, substrate bias, and heating and cooling conditions can bechanged in the same manner when performing sputtering or CVD. Thesurface of a vapor deposited substrate may be subjected to a coronadischarge treatment, a flame treatment, a low temperature plasmatreatment, a glow discharge treatment, a reverse sputtering treatment, asurface roughening treatment and the like, before or during vapordeposition with a metal or inorganic oxide, thereby still moreincreasing the adhesion strength of a metal or inorganic oxideeffectively.

In addition, the polyester film of the present invention can have aprinted ink layer formed on at least one surface thereof. As theprinting ink used for forming a printed ink layer is one generally used,such as an ink comprising a coloring material comprising a pigment ordye, a binder and a volatile organic solvent as constituent components.When light, particularly UV, is to be blocked with the printed inklayer, an ink layer having UV blocking property is formed. A printed inklayer can be formed by any method from gravure printing, offsetprinting, screen printing or other printing method, which is determinedaccording to the film thickness, size, continuous or sheet feeding, andthe like. Most generally, a printed ink layer is formed on a continuousfilm by gravure printing or offset printing.

In general, after forming a printed ink layer on a polyester film, athermally adhesive resin layer is laminated or applied on a surfaceopposite to the surface in contact with the polyester film of theprinted ink layer. After printing, therefore, a method comprisingcontinuously laminating a thermally adhesive resin layer is mostefficient and advantageous in terms of cost.

The polyester film of the present invention can be effectively utilizedfor use for which a nylon film is conventionally employed, which isspecifically a food packaging material requiring pinhole resistance andbag breakage resistance, particularly, a food packaging material forfish processed products involving a boiling treatment or a retorttreatment, pickles, daily dishes, livestock meat processed products andthe like, based on the superiority of polyester in the heat resistanceand moisture absorption dimensional stability. Moreover, it can beeffectively utilized as packaging for industrial materials such asinfusion pack, semiconductor, pet food, agricultural chemicals,fertilizers, precision equipment and the like, as well as medical,electronic, agricultural, mechanical products and the like. In addition,it can be effectively utilized as a material of packaging involvingvacuum forming and air pressure forming, such as molded containers andthe like, and a material of cards and electronic equipment cases, takingadvantage of heat resistance, impact resistance and crystallizationproperties.

EXAMPLES

The present invention is explained in detail in the following byreferring to Examples. Each property value in the present invention wasmeasured as shown below.

1. Reduced Viscosity

Reduced viscosity (ηsp/c)

A polymer (0.125 g) was dissolved in phenol/tetrachloroethane=6/4(weight ratio) (25 mL) and measurement was performed at 25° C. with aUbbelohde viscometer. The unit was dL/g. The resin was in the form ofchips, and the film was cut finely and used for the measurement.

2. Initial Elastic Modulus

The measurement was performed according to JIS-K7127-1989 usingAutograph (manufactured by Shimadzu Corporation: AG-5000 A).

3. Thermal Shrinkage

A sample was cut into 10 mm×150 mm, and gauge lines were marked at 100mm intervals on 10 sample pieces. They were left standing in a gear ovenat 150° C. for 30 min. without a load, after which they were taken outand the distance between gauge lines was measured at room temperature.The values were determined according to the following formula and anaverage value of 10 sample pieces was taken as the thermal shrinkage (%)of each sample.thermal shrinkage=((A−B)/A)×100

A: distance between gauge lines before heating

B: distance between gauge lines after heating

4. Difference in Thermal Shrinkage

An absolute value (%) of the difference between the values of thermalshrinkage in the longitudinal direction of the film and that in thetransverse direction thereof was taken as the difference in the thermalshrinkage.difference in thermal shrinkage=|C−D|

C: thermal shrinkage in the longitudinal direction

D: thermal shrinkage in the transverse direction

5. Haze

The measurement was performed based on JIS-K-7105-1981 and using aturbidity meter (manufactured by Nippon Denshoku Industries Co., Ltd.:NDH2000), and the value of haze (HZ) shown thereon was used.

6. Impact Strength

Using a film impact tester (manufactured by Toyo Seiki Seisaku-sho,LTD.: serial number T-84-3), a measurement film was pressed with aclamp, thrust a ½ inch diameter hemisphere impacting head thereinto, andthe impact strength of the sample was measured. Ten film samples wereprepared, and the impact strength was measured for 5 films at a time,changing the surface to be subjected to the impact. The sample was cutinto 100 mm×100 mm or more, and the ring that pressed the sample had aninner diameter of 30 mm. Average values of the impact strength of samplepieces were determined, and converted to those per 1 mm of thickness togive the impact strength (J/mm) of the film.

7. Gas Barrier Property

As a vapor deposition source, particles (about 3-5 mm in size) of Al₂O₃(purity 99.5%) and SiO₂ (purity 99.9%) were used. An adhesionmodification layer having a solid content of 0.3 g/m² (layer formed byapplying a mixture of a polyester resin aqueous dispersion (100 parts byweight, manufactured by Toyo Boseki Kabushiki. Kaisha: MD1200: solidcontent 30 wt %), methyl melamine (40 parts by weight, manufactured bySumitomo Chemical Co., Ltd.: M-30W), water (410 parts by weight) andisopropyl alcohol (50 parts by weight) with a wire bar and drying at160° C. for 1 min) was formed on only one surface of the polyester filmsobtained in Examples and Comparative Examples. The films were fed to avacuum vapor deposition apparatus. The inside of the chamber wasmaintained at a pressure of 1.5×10⁻⁵ Torr, and a mixture of inorganicoxides of SiO₂ (70 wt %) and Al₂O₃ (30 wt %) was evaporated by electronbeam heating (15 kw) to form a colorless, transparent vapor-depositedinorganic layer having a thickness of 220 Å on the adhesion modificationlayer.

Thereafter, the oxygen transmission rate was measured using an oxygentransmission rate measurement apparatus (manufactured byModernContorols: OX-TRAN 10/50A) at humidity 50%, temperature 25° C.,wherein the unit was mL/(m²·MPa·24 hours). The water vapor transmissionrate was measured using a water vapor transmission rate measurementapparatus (manufactured by ModernContorols: PERMATRAN) at humidity 0%,temperature 25° C., wherein the unit was g/(m²·24 hours). In view offood packaging, an oxygen level of not more than 50 mL/(m²·MPa·24 hours)was accepted and water vapor of not more than 5.0 g/(m²·24 hours) wasaccepted.

8. Gas Barrier Property After Boiling Treatment

The samples (10 sheets, 15 cm×15 cm) having a vapor-deposited layerformed by the method of the above-mentioned (7.) were prepared, andimmersed in hot water at 95° C. for 30 min. with stirring in a 3 Lcontainer. The samples were left standing at 23° C., 65 RH% for 24 hrand the oxygen transmission rate and the water vapor transmission ratewere measured in the same manner as in (6.). In view of food packaging,an oxygen level of not more than 50 mL/(m²·MPa·24 hours) was acceptedand water vapor of not more than 10 g/(m²·24 hours) was accepted.

9. Gas Barrier Property After Retort Treatment

A sample having a vapor-deposited layer formed by the method of theabove-mentioned (7.) was treated in an autoclave at 125° C. for 30 min.,left standing at 23° C., RH 65% for 24 hr and the oxygen transmissionrate and the water vapor transmission rate were measured in the samemanner as above. In view of food packaging, an oxygen level of not morethan 50 mL/(m²·MPa·24 hours) was accepted and water vapor of not morethan 10 g/(m²·24 hours) was accepted.

10. Whitening After Retort Treatment

After the test of 8, a fine appearance free of whitening was marked with◯ and whitened appearance with insufficient transparency was marked with×.

11. Vision of Printed Matter

Using a three color gravure printer manufactured by Modern Machinery,Ltd., blue, red and white of gravure ink “UNIVURE A” manufactured byDainippon Ink and Chemicals Incorporated were sequentially printed onone surface of the obtained polyester film having a vapor-depositedlayer or coating layer by gravure printing, and when the vision from theback was clear, ◯ was indicated and when the vision was not clear, × wasindicated. The printing was performed at a rate of 50 m/min, dryingtemperature 90° C.

12. Gelbo Test Evaluation

In a Gelbo-Flex tester (manufactured by TESTER SANGYO CO., LTD., serialnumber 27793), the ambient temperature was set to 23° C., a sample film(distance between chucks 178 mm, diameter 89 mmφ) was subjected torepetitive (1000 cycles) strain under the conditions of twisting angle440°, stroke length 155 mm, and the number of pinholes after thetwisting treatment was counted (the number of ink penetration on thefilter paper was counted). Five sample pieces were subjected to themeasurement, and the numbers of the obtained pinholes were averaged andthe average pinhole number was taken as the number of pinholes, based onwhich the pinhole resistance was compared.

13. Bag Dropping Test

As a sealant film, a nonoriented polypropylene film (manufactured byToyo Boseki Kabushiki Kaisha: P1153: 50 μm) was dry laminated on theobtained polyester film, and four sides were sealed with an impulsesealer. Ten bags (150 mm×150 mm) containing water were produced,subjected to a retort treatment at 125° C. for 30 min. The bags weredropped 20 times from a height of 1 m at 5° C. The average number ofdroppings before incidence of bag breakage or water leakage wasemployed. Preferably, the number is not less than 10, more preferablynot less than 15. When it is less than practical problems are caused inthe transportation of packages.

14. Boiling Test

As a sealant film, a nonoriented polypropylene film (manufactured byToyo Boseki Kabushiki Kaisha: P1153: 50 μm) was dry laminated on theobtained polyester film, and four sides were sealed with an impulsesealer. Bags (150 mm×150 mm) containing water were produced, immersed inhot water at 95° C. for 30 min. The presence or absence of bag breakageand defective appearance of the bag surface (wrinkle, whitening) wasevaluated.

15. Retort Test

As a sealant film, a nonoriented polypropylene film (manufactured byToyo Boseki Kabushiki Kaisha: P1153: 50 μm) was dry laminated on theobtained polyester film, and four sides were sealed with an impulsesealer. Bags (150 mm×150 mm) filled with water were produced, subjectedto a retort treatment at 120° C. for 30 min. The presence or absence ofbag breakage and defective appearance of the bag surface (wrinkle,whitening) was evaluated.

Example 1

Polyethylene terephthalate resin (A1) (reduced viscosity 0.75)comprising 2000 ppm of silicon dioxide (F) (manufactured by FUJI SILYSIACHEMICAL LTD.: Silysia 310) previously added during polymerization wasprepared as resin A, polybutylene terephthalate resin (B1) (reducedviscosity 1.20) and polybutylene terephthalate resin (B2) (reducedviscosity 1.10) comprising 1% of an organic phosphorus compound(manufactured by ASAHI DENKA Co., Ltd.: Adeka Stub PEP-45) were preparedas resin B, and polyester polymer (C1) which is terephthalicacid/sebacic acid//ethylene glycol/1,4-butanediol (90/10//60/40 (molarratio), molecular weight 200.0) previously. comprising 2% of talc (D)(average particle size by electron microscope method 3.5 μm) duringpolymerization was prepared as polyester C. They were cast into a singlescrew extruder (screw 65φ: UB manufactured by MITSUBISHI HEAVYINDUSTRIES, LTD.) at A1/B1/B2/C1=40/56/2/2 (parts by weight). Fortemperature setting of the extruder, temperatures of a feeding part(Ex1), a compressing part (Ex2), a measuring part (Ex3), the flow pathup to a filter, the filter part, the flow path up to a die, and the dieof the extruder were set, where Ex1 was 240° C., from Ex2 to the filterpart was 260° C., and thereafter was 255° C., and resins were supplied.The temperature of the resins measured immediately after extrusion fromthe T-die was 258° C. A 200 mesh filter was used. The resins extrudedfrom the T-die was rapidly cooled on a roll cooled to 20° C. accordingto an electrostatic adhesion method to give a non-oriented sheet havinga thickness of about 200 μm. The sheet was supplied to a roll drawingmachine, and drawn 3.3-fold in the longitudinal direction at 63° C.Subsequently, the sheet was transversely drawn 3.5-fold at 90° C. in atenter, and heat set in situ at 210° C. in the tenter while relaxing by3% in the transverse direction. The film was further led to rolls havinga rotation speed difference, and heat set at 220° C. in the tenter whilerelaxing by 3% in the longitudinal direction to give a film having athickness of 21 μm. The properties of the obtained film are shown inTable 1.

In addition, an adhesion modification layer was formed on one surface ofthe obtained polyester film by the method described in theaforementioned “7. Gas barrier property” to give a polyester filmhaving, on its surface, a colorless and transparent vapor-depositedlayer made of a mixed inorganic oxide of SiO₂ and Al₂O₃ and having athickness of 220 Å. The properties of the obtained film were evaluatedand are shown in Table 2.

Comparative Example 1

In the same manner as in Example 1 except that, after heat setting inthe transverse direction, the rate of relaxation in the longitudinaldirection using rolls having a rotation speed difference was set to0.02%, a polyester film and a polyester film having a vapor-depositedlayer were obtained. The properties of the obtained films were evaluatedand the results are shown in Table 1 and Table 2.

Example 2

In the same manner as in Example 1 except that, after heat setting inthe transverse direction, the rate of relaxation in the longitudinaldirection using rolls having a rotation speed difference was set to 2%,a polyester film and a polyester film having a vapor-deposited layerwere obtained. The properties of the obtained films were evaluated andthe results are shown in Table 1 and Table 2.

Example 3

In the same manner as in Example 1 except that, after heat setting inthe transverse direction, the rate of relaxation in the longitudinaldirection using rolls having a rotation speed difference was set to 4%,a polyester film and a polyester film having a vapor-deposited layerwere obtained. The properties of the obtained films were evaluated andthe results are shown in Table 1 and Table 2.

Comparative Example 2

In the same manner as in Example 1 except that, after heat setting inthe transverse direction, the rate of relaxation in the longitudinaldirection using rolls having a rotation speed difference was set to 10%,production of a polyester film was tried. However, wrinkles weredeveloped on the film during film formation, and the property valuescould not be measured.

Example 4

In the same manner as in Example 1 except that the mixing ratio ofrespective resins was set to A1/B1/B2/C1 75/21/2/2 (parts by weight), apolyester film and a polyester film having a vapor-deposited layer wereobtained. The properties of the obtained films were evaluated and theresults are shown in Table 1 and Table 2.

Example 5

In the same manner as in Example 1 except that the mixing ratio ofrespective resins was set to A1/B1/B2/C1 25/71/2/2 (parts by weight), apolyester film and a polyester film having a vapor-deposited layer wereobtained. The properties of the obtained films were evaluated and theresults are shown in Table 1 and Table 2.

Example 6

In the same manner as in Example 1 except that polytrimethyleneterephthalate having a reduced viscosity of 0.83 was used instead ofpolybutylene terephthalate, a polyester film and a polyester film havinga vapor-deposited layer were obtained. The properties of the obtainedfilms were evaluated and the results are shown in Table 1 and Table 2.

Comparative Example 3

In the same manner as in Example 1 except that the rate of thelongitudinal orientation was set to 2.5-fold and the rate of thetransverse orientation was set to 2.5-fold, a polyester film and apolyester film having a vapor-deposited layer were obtained. Theproperties of the obtained films were evaluated and the results areshown in Table 1 and Table 2.

Comparative Example 4

In the same manner as in Example 1 except that the temperature of Ex1,Ex2, Ex3 and the flow path up to the filter, the filter part, the flowpath up to the die, and the die of the extruder was set to 290° C. forall of them, a polyester film and a polyester film having avapor-deposited layer were obtained. The properties of the obtainedfilms were evaluated and the results are shown in Table 1 and Table 2.TABLE 1 initial elastic thermal reduced modulus (GPa) shrinkage (%)difference (%) in thermal impact appearance viscosity (longitudinal/(longitudinal/ shrinkage strength haze after (dL/g) transverse)transverse) |longitudinal-transverse| (J/mm) (%) printing Ex. 1 0.852.7/2.8 0.5/0.3 0.2 65 3.8 ◯ Ex. 2 0.84 2.8/2.9 0.8/0.4 0.4 66 3.2 ◯ Ex.3 0.84 2.8/2.9 0.4/0.2 0.2 65 3.2 ◯ Ex. 4 0.89 3.8/4.2 0.9/0.3 0.6 633.9 ◯ Ex. 5 0.97 3.0/3.1 0.5/0.4 0.1 72 3.8 ◯ Ex. 6 0.85 3.1/3.2 0.7/0.50.2 60 3.2 ◯ Comp. 0.84 2.7/3.1 2.5/0.5 2.0 62 3.5 ◯ Ex. 1 Comp. 0.84Wrinkles were developed during film formation, and measurement wasunavailable. Ex. 2 Comp. 0.84 1.5/1.5 1.3/1.1 0.2 45 3.5 X printing Ex.3 displacement occurred Comp. 0.71 2.3/2.2 0.8/0.4 0.4 35 3.2 ◯ Ex. 4

TABLE 2 thermal gas barrier property of shrinkage (%) polyester film(before gas barrier property after vapor haze (%) bag retort treatment)after retort treatment deposition after drop oxygen water vapor oxygenwater vapor (longitudinal/ retort test transmission transmissiontransmission transmission transverse) treatment whitening (times) raterate rate rate Ex. 1 0.3/0.1 4.1 ◯ 17 15 1.5 17 3.3 Ex. 2 0.3/0.1 3.3 ◯17 15 1.5 17 1.4 Ex. 3 0.3/0.1 3.4 ◯ 17 16 1.5 18 2.1 Ex. 4 0.3/0.1 3.9◯ 14 17 1.5 17 2.1 Ex. 5 0.3/0.1 3.9 ◯ 17 12 1.5 18 2.2 Ex. 6 0.3/0.13.3 ◯ 17 17 1.5 19 1.9 Comp. 1.4/0.2 3.6 ◯ 15 21 2.6 178 not less Ex. 1than 20 Comp. Wrinkles were developed during film formation, andmeasurement was unavailable. Ex. 2 Comp. 0.2/0.2 3.6 ◯ 16 21 2.6 25 5.6Ex. 3 Comp. 0.3/0.1 3.3 X 3 15 1.5 178 not less Ex. 4 than 20

Example 7

(Preparation of Coating Solution)

A polyester aqueous dispersion (100 parts by weight, manufactured byToyo Boseki Kabushiki Kaisha: MD1200, solid content 30 wt %), methylmelamine (40 parts by weight, manufactured by Sumitomo Chemical Co.,Ltd.: M-30W), 20 wt % aqueous dispersion (20 parts by weight) ofcolloidal silica particles (manufactured by Nissan Chemical Industries,Ltd.: SnowTex OL, average particle size 40 nm), water (410 parts byweight), and isopropyl alcohol (50 parts by weight) were mixed to give acoating solution having a solid content concentration of 10%.

(Formation of Film)

Polyethylene terephthalate resin (A1) (reduced viscosity 0.75)comprising 2000 ppm of silicon dioxide (F) (manufactured by FUJI SILYSIACHEMICAL LTD.: Silysia 310) previously added during polymerization wasprepared as resin A, polybutylene terephthalate resin (B1) (reducedviscosity 1.20) and polybutylene terephthalate resin (B2) (reducedviscosity 1.10) comprising 1% of an organic phosphorus compound(manufactured by ASAHI DENKA Co., Ltd.: Adeka Stub PEP-45) were preparedas resin B, and polybutylene terephthalate (C1) (copolymerized monomers:terephthalic acid/sebacic acid//ethylene glycol/1,4-butanediol)(90/10//60/40 (molar ratio) molecular weight 2000) previously comprising2% of talc (D) (average particle size by electron microscope method 3.5μm) during polymerization was prepared as polyester C. They were castinto a single screw extruder (screw 65φ: UB manufactured by MITSUBISHIHEAVY INDUSTRIES, LTD.) at A1/B1/B2/C1=40/56/2/2 (parts by weight). Fortemperature setting of the extruder, temperatures of a feeding part(Ex1), a compressing part (Ex2), a measuring part (Ex3), the flow pathup to a filter, the filter part, the flow path up to a die, and the dieof the extruder were set, where Ex1 was 240° C., from Ex2 to the filterpart was 260° C., and thereafter was 255° C., and resins were supplied.The temperature of the resins measured immediately after extrusion fromthe T-die was 258° C. A 200 mesh filter was used. The resins extrudedfrom the T-die was rapidly cooled on a roll cooled to 20° C. accordingto an electrostatic adhesion method to give a non-oriented sheet havinga thickness of about 200 μm. The sheet was supplied to a roll drawingmachine, and drawn 3.3-fold in the longitudinal direction at 63° C.Then, a coating solution was applied with a wire bar and dried byblowing hot air at 70° C. to the coated surface at 20 m/sec for 30 sec.Subsequently, the sheet was transversely drawn 3.6-fold at 88° C. in atenter, and set in situ in the tenter at 200° C. for about 10 sec and at220° C. for about 10 sec, while relaxing by 4% in the transversedirection to give an about 16 μm polyester film.

In addition, a polyester film having a colorless and transparentvapor-deposited layer made of a mixed inorganic oxide of SiO₂ and Al₂O₃and having a thickness of 220 Å was obtained, which layer was formed onthe surface of the easily adhesive coating layer (coated layer) of theobtained polyester film. The properties of the obtained film wereevaluated and are shown in Table 3.

Example 8

In the same manner as in Example 7 except that the mixing ratio ofrespective resins was set to A1/B1/B2/C1=75/21/2/2 (parts by weight), apolyester film and a polyester film having a vapor-deposited layer wereobtained. The properties of the obtained films were evaluated and theresults are shown in Table 3.

Example 9

In the same manner as in Example 7 except that the mixing ratio ofrespective resins was set to A1/B1/B2/C1 25/71/2/2 (parts by weight), apolyester film and a polyester film having a vapor-deposited layer wereobtained. The properties of the obtained films were evaluated and theresults are shown in Table 3.

Example 10

In the same manner as in Example 7 except that the coating solution wasdried by blowing hot air at 90° C. to the coated surface at 15 m/sec for30 sec, a polyester film and a polyester film having a vapor-depositedlayer were obtained.

The properties of the obtained films were evaluated and the results areshown in Table 3.

Comparative Example 5

In the same manner as in Example 7 except that the coating solution wasdried by blowing hot air at 110° C. to the coated surface at 15 m/secfor 20 sec, a polyester film and a polyester film having avapor-deposited layer were obtained. The properties of the obtainedfilms were evaluated and the results are shown in Table 3.

Comparative Example 6

In the same manner as in Example 7 except that the coating solution wasdried by blowing hot air at 70° C. to the coated surface at 35 m/sec for30 sec, a polyester film and a polyester film having a vapor-depositedlayer were obtained. The properties of the obtained films were evaluatedand the results are shown in Table 3.

Example 11

In the same manner as in Example 9 except that the coating solution wasprepared as shown in the following, a polyester film and a polyesterfilm having a vapor-deposited layer were obtained.

(Preparation of Polyester Copolymer)

Dimethyl terephthalate (345 parts), 1,4-butanediol (211 parts), ethyleneglycol (270 parts) and tetra-n-butyl titanate (0.5 part) were charged ina stainless steel autoclave equipped with a stirrer, a thermometer and apartial reflux condenser, and transesterification reaction was carriedout from 160° C. to 220° C. over 4 hr. Then, fumaric acid (14 parts) andsebacic acid (160 parts) were added, and the temperature was raised from200° C. to 220° C. over 1 hr to carry out esterification reaction. Thetemperature was raised to 255° C., the reaction system was graduallydepressurized, and the reaction was carried out under reduced pressureof 0.22 mmHg for 1.5 hr to give a polyester copolymer. The obtainedpolyester was pale-yellow and transparent and had a weight averagemolecular weight of 20000.

(Self-Crosslinking Polyester Graft Copolymer)

The above-mentioned polyester copolymer resin (75 parts), methyl ethylketone (56 parts) and isopropyl alcohol (19 parts) were charged in areaction vessel equipped with a stirrer, a thermometer, a refluxcondenser and a titration dropping device, and the resins were dissolvedby heating and stirring at 65° C. After complete dissolution of theresins, maleic anhydride (15 parts) was added to the polyester solution.Then, styrene (10 parts), and a solution of azobisdimethylvaleronitrile(1.5 parts) in methyl ethyl ketone (12 parts) were added dropwise to thepolyester solution at 0.1 mL/min, and the mixture was further stirredfor 2 hr. The reaction solution was sampled for analysis, and methanol(5 parts) was added. Then, water (300 parts) and triethylamine (15parts) were added to the reaction solution, and the mixture was stirredfor 1 hr. Thereafter, the inside temperature of the reaction vessel wasraised to 100° C., methyl ethyl ketone, isopropyl alcohol and excesstriethylamine were distilled away to give an aqueously dispersinggraft-polymerization resin. The aqueously dispersing graft resin waspale-yellow and transparent and had a glass transition temperature of−10° C.

(Preparation of Coating Solution)

The aqueously dispersing graft resin and, as a polyester resin, anaqueous polyester resin obtained by copolymerization of terephthalicacid/isophthalic acid/5-sulfoisophthalic acid/ethyleneglycol/1,4-butanediol at 25/20/5/25/25 (weight ratio) were diluted withwater:isopropyl alcohol=9:1 (weight ratio) to a weight ratio of 90:10,solid content concentration 10% and used as a coating solution. Theproperties of the obtained film were evaluated. The results thereof areshown in Table 3.

Example 12

In the same manner as in Example 9 except that polytrimethyleneterephthalate resin having a reduced viscosity of 0.83 was used insteadof polybutylene terephthalate resin, a polyester film and a polyesterfilm having a vapor-deposited layer was obtained. The properties of theobtained film were evaluated. The results thereof are shown in Table 3.

Comparative Example 7

In the same manner as in Example 9 except that a coating solution wasnot applied, a substrate film for forming a polyester film and a filmhaving a vapor-deposited layer were obtained. The properties of theobtained film were evaluated. The results thereof are shown in Table 3.

Comparative Example 8

In the same manner as in Example 7 except that the longitudinalorientation ratio was set to 2.5-fold and the transverse orientationratio was set to 2.5-fold, a polyester film and a film having avapor-deposited layer were obtained. The properties of the obtained filmwere evaluated. The results thereof are shown in Table 3.

Comparative Example 9

In the same manner as in Example 7 except that the temperatures of Ex1,Ex2, Ex3 and the flow path up to the filter, the filter part, the flowpath up to the die, and the die of the extruder was set to 290° C. forall of them, a polyester film and a film having a vapor-deposited layerwere obtained.

The properties of the obtained film were evaluated. The results thereofare shown in Table 3. TABLE 3 gas barrier gas barrier property initialproperty after after vapor elastic thermal vapor deposition deposition,boiling modulus shrinkage appear- oxygen water oxygen water bag reduced(GPa) (%) rance trans- vapor trans- vapor impact drop viscosity(longitudinal/ (longitudinal/ haze after mission transmission missiontransmission strength test (dL/g) transverse) transverse) (%) printingrate rate rate rate (J/mm) (times) Ex. 7 0.85 2.7/2.8 2.5/1.1 3.8 ◯ 122.5 24 3.3 71 17 Ex. 8 0.89 3.8/4.1 2.0/0.8 2.8 ◯ 10 1.0 12 1.4 70 18Ex. 9 0.97 3.1/3.2 1.9/0.7 2.0 ◯ 14 1.8 16 2.1 72 18 Ex. 10 0.94 3.2/3.21.8/0.6 2.9 ◯ 15 1.7 20 2.0 74 18 Ex. 11 0.95 3.0/3.2 1.9/0.8 2.9 ◯ 121.2 15 1.9 73 18 Ex. 12 0.85 3.3/3.5 2.1/0.8 1.8 ◯ 16 2.2 20 2.8 68 16Comp. 0.93 3.1/3.2 2.1/0.9 9.4 X 82 33.5 241 not less 73 17 Ex. 5 than20 Comp. 0.93 3.1/3.1 2.2/0.9 7.2 X 65 21.3 195 not less 71 17 Ex. 6than 20 Comp. 0.94 3.1/3.2 1.8/0.6 1.2 ◯ 35 4.2 380 not less 71 16 Ex. 7than 20 Comp. 0.85 1.6/1.6 1.4/0.8 1.8 X 16 2.2 20 2.8 88 17 Ex. 8 Comp.0.71 2.7/2.7 1.8/0.6 1.2 ◯ 35 4.2 380 not less 28 3 Ex. 9 than 20

Example 13

Polyethylene terephthalate resin A1 (reduced viscosity 0.75) comprising2000 ppm of silicon dioxide (F) (manufactured by FUJI SILYSIA CHEMICALLTD.: Silysia 310) previously added during polymerization as resin A andpolybutylene terephthalate resin B1 (reduced viscosity 1.20) as resin Bwere cast into a single screw extruder (65φ) at resin A1/resin B1=60/40(parts by weight). For temperature setting of the extruder, temperaturesof a feeding part (Ex1), a compressing part (Ex2), a measuring part(Ex3), the flow path up to a filter, the filter part, the flow path upto a die, and the die of the extruder were set, where Ex1 was 240° C.,from Ex2 to the filter part was 260° C., and thereafter was 255° C., andresins were supplied. The temperature of the resins measured immediatelyafter extrusion from the T-die was 258° C. A 200 mesh filter was used.The resins extruded from the T-die was rapidly cooled on a roll cooledto 20° C. according to an electrostatic adhesion to give a non-orientedfilm having a thickness of about 200 μm. The film was supplied to a rolldrawing machine, and drawn 3.3-fold in the longitudinal direction at 80°C. Subsequently, the sheet was transversely drawn 3.6-fold at 95° C. ina tenter, and heat set in situ at 200° C. for about 10 seconds and at210° C. for about 10 seconds in the tenter while relaxing by 6% in thetransverse direction to give a polyester film having a thickness ofabout 16 μm. The properties of the obtained film were evaluated. Theresults thereof are shown in Table 4.

Example 14

In the same manner as in Example 13 except that the starting materialwas resin A1/resin B1=85/15 (parts by weight), the temperatureconditions of extrusion step were set to 250° C. for Ex1, 270° C. fromEx2 to the filter part, and 255° C. thereafter, the orientationtemperature in the longitudinal direction was set to 110° C., and theorientation temperature in the transverse direction was set to 120° C.,a polyester film was obtained. The properties of the obtained film wereevaluated. The results thereof are shown in Table 4.

Example 15

In the same manner as in Example 13 except that the starting materialwas resin A1/resin B1=40/60 (parts by weight), the orientationtemperature in the longitudinal direction was set to 65° C., and theorientation temperature in the transverse direction was set to 85° C., apolyester film was obtained. The properties of the obtained film wereevaluated. The results thereof are shown in Table 4.

Example 16

In the same manner as in Example 13 except that the starting materialwas resin A1/resin B1=15/85 (parts by weight), the orientationtemperature in the longitudinal direction was set to 55° C., and theorientation temperature in the transverse direction was set to 75° C., apolyester film was obtained. The properties of the obtained film wereevaluated. The results thereof are shown in Table 4.

Comparative Example 10

In the same manner as in Example 13 except that polyethyleneterephthalate resin A2 (reduced viscosity 0.65) comprising 2000 ppm ofsilicon dioxide (F) (manufactured by FUJI SILYSIA CHEMICAL LTD.: Silysia310) previously added during polymerization was used as resin A andpolybutylene terephthalate resin B2 (reduced viscosity 0.75) was used asresin B and they were used at resin A2/resin B2=60/40 (parts by weight),a polyester film was obtained. The properties of the obtained film wereevaluated. The results thereof are shown in Table 4.

Comparative Example 11

In the same manner as in Example 13 except that the starting materialwas resin A1/resin B1=95/5 (parts by weight), the temperature conditionsof extrusion step were set to 250° C. for Ex1, 275° C. from Ex2 to thefilter part, and 255° C. thereafter, the orientation temperature in thelongitudinal direction was set to 110° C., and the orientationtemperature in the transverse direction was set to 125° C., a polyesterfilm was obtained. The properties of the obtained film were evaluated.The results thereof are shown in Table 4.

Comparative Example 12

In the same manner as in Example 13 except that the temperatureconditions of extrusion step were set to 280° C. from Ex2 to the filterpart, and 255° C. thereafter, and the heat setting treatment aftertransverse orientation was applied at 190° C. for about 10 seconds andat 195° C. for about 10 seconds, a polyester film was obtained. Theproperties of the obtained film were evaluated. The results thereof areshown in Table 4.

Comparative Example 13

In the same manner as in Example 13 except that the heat settingtreatment after transverse orientation was applied at 225° C. for about10 seconds and at 235° C. for about 10 seconds, a polyester film wasobtained. The properties of the obtained film were evaluated. Theresults thereof are shown in Table 4.

Comparative Example 14

In the same manner as in Example 13 except that, as the conditions ofthe extruder, the temperature was set to 285° C. from Ex2 to the filterpart and 2755° C. thereafter, a polyester film was prepared. Thetemperature of the resin immediately after T-die was 278° C. Theproperties of the obtained film were evaluated. The results thereof areshown in Table 4. TABLE 4 Comp. Comp. Comp. Comp. Comp. Ex. 13 Ex. 14Ex. 15 Ex. 16 Ex. 10 Ex. 11 Ex. 12 Ex. 13 Ex. 14 starting material A1 6085 40 15 — 95 60 60 60 polyester resin A2 — — — — 60 — — — — A3 40 15 6085 — 5 40 40 40 A4 — — — — 40 — — — — extruder compressing (° C.) 260270 260 260 260 275 280 260 285 part temperature die outlet resin (° C.)258 260 258 258 258 260 276 258 278 temperature heat setting (° C.)200/210 200/210 200/210 200/210 200/210 200/210 190/195 225/235 200/210treatment temperature reduced viscosity (dL/g) 0.84 0.76 0.89 1.05 0.710.73 0.74 0.84 0.72 initial elastic modulus (GPa) 3.4/3.6 3.8/3.93.2/3.1 2.6/2.8 3.3/3.4 4.2/4.5 3.4/3.6 3.5/3.7 3.6/3.7 (longitudinal/transverse) impact strength (J/mm) 52 47 55 59 33 43 44 35 38 haze (%)3.2 2.7 3.5 3.8 3.4 2.8 3.5 4.5 4.0 thermal shrinkage (%) 3.2/2.93.3/2.9 3.3/3.0 3.2/3.0 3.2/3.0 3.3/3.0 6.1/6.4 1.3/1.2 3.2/3.0 (150°C., 30 min (longitudinal/ transverse) Gelbo test evaluation (pinholes) 23 1 0 5 8 3 6 5 bag drop breakage (times) 17 18 17 18 8 9 13 5 6frequency boil evaluation (appearance) fine fine fine fine fine finecurled fine fine retort evaluation (appearance) fine fine fine finewhitened fine fine fine whitened gravure printing (appearance) fine finefine fine fine fine pitch fine fine evaluation lag

While the polyester film of the present invention has been describedbased on plural Examples in the above, the present invention is notlimited to the constitutions described in the Examples above, butrather, various changes may be made to the constitution as appropriatewithout departing from the gist of the present invention, such asappropriate combination of the constitutions described in respectiveExamples and the like.

INDUSTRIAL APPLICABILITY

As mentioned above, since the polyester film of the present invention ischaracteristically superior in mechanical strength, heat resistance,chemical resistance, insulation property and thermal dimensionalstability, it can be preferably used for the fields associated withboiling or retort treatment, which require tenacity, pinhole resistance,bending resistance, bag breakage resistance on dropping, impactresistance and the like, fields requiring thermoforming or vacuumforming, and uses such as packaging bags for water-containing food,pharmaceutical products and the like.

1. A polyester film having an initial elastic modulus in at least onedirection of 2.5-10 GPa, an impact strength of 40-10000 J/mm, a thermalshrinkage in at least one direction at 150° C. of −0.5% to 6% and a hazeof 0.001% to 7%.
 2. The polyester film of claim 1, which is made of apolyester resin composition comprising 10-90 wt % of polyethyleneterephthalate resin (A), and 90-10 wt % of a polybutylene terephthalateresin and/or polytrimethylene terephthalate resin (B).
 3. The polyesterfilm of claim 1, wherein the polyester film has a reduced viscosity ofnot less than 0.80.
 4. The polyester film of claim 1, wherein theabsolute value of the difference in the thermal shrinkage between thelongitudinal direction and the transverse direction of the substratefilm is not more than 1.1%.
 5. The polyester film of claim 1, whereinthe thermal shrinkage in the longitudinal direction and the transversedirection at 150° C. of the substrate film is each 0% to 4%.
 6. Thepolyester film of claim 1, wherein the number of pinholes formed bybending the substrate film 1000 times at 23° C. in a Gelbo-Flex test isnot more than
 5. 7. The polyester film of claim 1, wherein at least onesurface of the film has at least one surface treatment layer selectedfrom a coating layer, a corona discharge treatment layer, avapor-deposited metal layer, a vapor-deposited inorganic oxide layer andan ink printed layer.
 8. The polyester film of claim 7, wherein theeasily adhesive coating layer is composed of a coating solutioncomprising at least binder (C) and hardener (D).
 9. The polyester filmof claim 7, which is obtained by applying a coating solution for formingthe aforementioned easily adhesive coating layer, and then subjectingthe resulting film to at least uniaxial orientation.
 10. The polyesterfilm of claim 1, which is used as a packaging material.
 11. Thepolyester film of claim 2, wherein the polyester film has a reducedviscosity of not less than 0.80.
 12. The polyester film of claim 2,which is used as a packaging material.
 13. The polyester film of claim3, which is used as a packaging material.
 14. The polyester film ofclaim 4, which is used as a packaging material.